Syntheses and crystal structures of four 4-(4-methoxyphenyl)piperazin-1-ium salts: trifluoroacetate, 2,3,4,5,6-pentafluorobenzoate, 4-iodobenzoate, and a polymorph with 4-methylbenzoate

The syntheses and low-temperature (90 K) crystal structures of four organic salts of 4-(4-methoxyphenyl)piperazin-1-ium are presented.


Structural commentary
Three of the four salts (see Figs. 1-4) crystallized as monohydrates; only I is anhydrous. Structure III, which is a much higher quality, low-temperature re-investigation of CSD entry KUJPUD [Kiran Kumar et al., 2020b; CSD = Cambridge Structural Database (Groom et al., 2016)] contains three copies of the cation, anion, and water within its asymmetric unit (i.e. Z 0 = 3), all others have Z 0 = 1. Structure IV is a polymorph of CSD entry XEMCIF (Shankara Prasad et al., 2022). The asymmetric units were chosen so as to make the N-HÁ Á ÁO hydrogen-bond geometry between the cation and anion as similar as possible, i.e. with the equatorial H atom of the NH 2 + group as donor (see section 3: Supramolecular    features). The overall conformations of the cations in I-IV are determined, in large part, by the twist of the N2-C5 bonds that connect the 4-methoxyphenyl and piperazinium groups . These twists, quantified for example by the dihedral angle between the mean planes of the benzene ring (C5-C10) and the four carbon atoms (C1-C4) of the piperazinium rings, are 40.63 (5) (I), 36.05 (4) (II), 25.28 (13), 26.59 (12), and 24.82 (11) (for IIIa,b,c, respectively), and 7.57 (8) (IV), showing moderate variability across the four structures (Fig. 5). The geometry of the N2 atoms in each cation is nonplanar; the sums of bond angles about N2 ranging from 337.46 (16) in I to 342.4 (3) for N2C in IIIc. In each structure, the 4-methoxy groups are close to coplanar with their attached benzene rings, the largest deviation out of plane being only 0.188 (4) Å for C11C in IIIc, which corresponds to a C9C-C8C-O1C-C11C torsion angle of 172.3 (2) . The conformation of the trifluoroacetate anion in I, is largely unremarkable, having a dihedral angle between the plane of the carboxylate group and the plane formed by atoms C12, C13, and F3 of 89.29 (12) . In the substituted benzoate anions of II, III, and IV, the dihedral angles between the carboxylate groups and the benzene rings are 43.28 (5) (II), 3.8 (2) , 7.46 (19) , and 23.6 (2) (IIIa,b,c, respectively) and 8.60 (11) (IV).

Supramolecular features
Strong hydrogen bonds are the dominant intermolecular interactions in each of the four salts. All other hydrogen-bondtype interactions are weak. Salt II has weakinteractions and III has IÁ Á ÁI contacts, as described below.
The hydrogen bonding in I is the simplest of the four salts. There are only two N-HÁ Á ÁO hydrogen bonds, the shortest being N1-H1NAÁ Á ÁO2, at d DÁ Á ÁA = 2.7084 (13) Å . In addition, N1-H1NBÁ Á ÁO3 i (symmetry code as per Table 1) at d DÁ Á ÁA = 2.8329 (14) Å , connects cations and anions into chains that extend parallel to its crystallographic b axis, with inversionrelated chains running anti-parallel (Fig. 6). Other than a few C-HÁ Á ÁO and C-HÁ Á ÁF close contacts (also listed in Table 1), there are no other significant inter-species interactions.

Figure 5
An overlay of six 4-MeOPP cations (least-squares fit of piperazinium ring atoms), showing the variability of MeOPP conformation across structures I-IV.

Synthesis and crystallization
All reagents were obtained commercially and were used as received. For the synthesis of the salts, equimolar quantities (0.52 mmol of each component) of N-(4-methoxyphenyl)piperazine (100 mg) (from Sigma-Aldrich) and either trifluoroacetic acid (60 mg, I), pentafluorobenzoic acid (110 mg, II), 4-iodobenzoic acid (129 mg, III), or 4-methylbenzoic acid (71 mg, IV) were separately dissolved in methanol (10 ml). The two solutions were mixed and stirred briefly at 333 K and then set aside to crystallize, giving the solid products I to IV after a few days. The products were collected by filtration and then dried in air (I: yield 80%, m.p. 390-392 K; II: yield 75%, m.p. 375-377 K; III: yield 85%, m.p. 426-428 K; IV: yield 70%, m.p. 406-408 K). Crystals of compounds I to IV suitable for single-crystal X-ray diffraction were grown by slow evaporation, at ambient temperature and in the presence of air, of solutions in methanol:ethyl acetate (initial composition 1:1, v/v).

4-(4-Methoxyphenyl)piperazin-1-ium 2,2,2-trifluoroacetate (I)
Crystal data   (7) Special details Experimental. The crystal was mounted using polyisobutene oil on the tip of a fine glass fibre, which was fastened in a copper mounting pin with electrical solder. It was placed directly into the cold gas stream of a liquid-nitrogen based cryostat (Hope, 1994;Parkin & Hope, 1998). Diffraction data were collected with the crystal at 90K, which is standard practice in this laboratory for the majority of flash-cooled crystals. Geometry. All esds (except the esd in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell esds are taken into account individually in the estimation of esds in distances, angles and torsion angles; correlations between esds in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell esds is used for estimating esds involving l.s. planes. Refinement. Refinement progress was checked using Platon (Spek, 2020) and by an R-tensor (Parkin, 2000). The final model was further checked with the IUCr utility checkCIF.

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å 2 )
x 0.66484 (7) 0.5639 (2) 0.57944 (4) 0.0179 (2) (3) Atomic displacement parameters (Å 2 ) 0.0169 (5) 0.0209 (6) (13) Hydrogen-bond geometry (Å, º)   (14) Special details Experimental. The crystal was mounted using polyisobutene oil on the tip of a fine glass fibre, which was fastened in a copper mounting pin with electrical solder. It was placed directly into the cold gas stream of a liquid-nitrogen based cryostat (Hope, 1994;Parkin & Hope, 1998). Diffraction data were collected with the crystal at 90K, which is standard practice in this laboratory for the majority of flash-cooled crystals. Geometry. All esds (except the esd in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell esds are taken into account individually in the estimation of esds in distances, angles and torsion angles; correlations between esds in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell esds is used for estimating esds involving l.s. planes. Refinement. Refinement progress was checked using Platon (Spek, 2020) and by an R-tensor (Parkin, 2000

Special details
Experimental. The crystal was mounted using polyisobutene oil on the tip of a fine glass fibre, which was fastened in a copper mounting pin with electrical solder. It was placed directly into the cold gas stream of a liquid-nitrogen based cryostat (Hope, 1994;Parkin & Hope, 1998). Diffraction data were collected with the crystal at 90K, which is standard practice in this laboratory for the majority of flash-cooled crystals.